What Radio Readers Want to Know 



Interesting and Instructive Questions and Answers 



Dimensions for a Receiving Tuner; Effect 

 of Variometer on Wavelength 



E. C. S., Deer Lodge, Montana, writes: 



Q. 1. Please give the dimensions for a 4,000- 

 meter inductively coupled receiving tuner. The 

 secondary winding is to be shunted by a conden- 

 ser of .0005 microfarads and the primary by one 

 of .001 microfarads. The aerial has a natural 

 wavelength of 450 meters, 



A. 1. The fact that the aerial has a natural 

 wavelength of 450 meters does not gi\ e us suffi- 

 cient basis to compute accurately the dimensions 

 of the primary' winding. We must know the 

 inductance and capacity of the aerial system to 

 work out the problem. The secondary winding 

 may be wound on a form 4 in. in diameter, 7 in. in 

 length with No. 32 S.S.C. wire. The primary 

 winding should be 42 in. in diameter, 6 in. in 

 length, wound closely with No. 24 S.S.C. wire. 



Q. 2. How many meters will the variometer 

 described on page 539 of the October, 1915, issue 

 of the PoPUL.\R Science Monthly add to the 

 wavelength of a receiving circuit? 



A. 2. We would require more details of the 

 particular circuit in which it is to be employed to 

 answer this question definitely; but off-hand we 

 advise that with the No. 20 wire recommended in 

 that issue it will have but a slight effect on the 

 tuning of a circuit. To be eflectual it should be 

 wound with No. 30 S.S.C. wire and will then alter 

 the wavelength of a small set about 250 meters. 



Making a Transmitter for an 

 Amateur Station 



C. F. L., Galveston, Texas, writes: 



Q. 1. Please give the data for the construc- 

 tion of a J-K.W. open core transformer to be 

 operated on 300 volts alternating current at a 

 frequency of 500 cycles. The secondary winding 

 is to deliver 20,000 volts. 



A. 1. Data for an open core transformer is 

 not available at this writing but it may be pos- 

 sible to supply it at a later date. The following, 

 however, is applicable to a closed core trans- 

 former. The core is 9 in. in length, 2 1/16 in. in 

 width and ij in. in thickness. The ends are 

 5} in. in length and of the same thickness. 

 The primary winding has 98 turns of No. 10 

 D.C.C. wire wound in two layers. The second- 

 ary win<ling is made in sections and has totally 

 4<x)0 turns of No. 20 D.C.C. wire. The secondary 

 winding should be split into 5 sections wound 

 cither in the form of pancakes or multilayered 

 units of 36 layers each. Appropriate insulation 

 between the windings and the core is required. 



Q. 2. Give the dimensions for an oil- 

 immersed condenser to be used in connection 



with the above transformer. I prefer to 

 use photographic plates 8 in. by 10 in. if possible, 

 and should like to have two sections of con- 

 densers in series. I propose to use a syn- 

 chronous rotary spark-gap with this set. 



A. 2. We presume that you desire to operate 

 the station at the wavelength of 200 meters and 

 consequently the capacity of this condenser can- 

 not exceed .01 microfarads. If the 8 in. by 10 in. 

 photographic plates are covered with tinfoil 6 in. 

 by 8 in. each plate will have an approximate 

 capacity of .00066 microfarads and therefore 16 

 plates connected in parallel will give about the 

 required value of capacity. Since you prefer a 

 series parallel connection, you must connect 32 

 plates in parallel in each bank and then connect 

 the two banks in series. 



Q. 3. In view of the fact that my aerial is so 

 small, would not a high voltage set of this 

 character carry further than a j-K.W. set using a 

 voltage of 7,000 and a quenched spark-gap of 

 poor design? 



A. 3. Yes, by all means. The higher poten- 

 tial will enable you to use a greater amount of 

 power with the restricted condenser which the 

 2(K)-meter wave requires. The fundamental 

 wavelength of your antenna system is about 215 

 meters and can be reduced to 200 meters by con- 

 necting a "short wave condenser" in series with 

 the antenna system, or, preferably, by attaching 

 the lead-in wires to the center of the flat top 

 portion. 



A Long-Wave Tuner 



J. L., Scranton, Pa. 



Q. 1. Where can I obtain the parts and full 

 directions for constructing a 15000-meter in- 

 ductive coupler of the Navy type, including 

 all the blueprints and necessary diagram of 

 connections ? 



A. 1. We know of no concern which supplies 

 such data and parts. If you ha\e become 

 fann'liar with the construction and operation of 

 smaller receiving transformers you should have 

 no dilViculty in building an apparatus of the sort 

 you wish. The exact dimensions will of course 

 depend upon the size of antenna to be used. For 

 a good-sized aerial, your primary-coil should be 

 of No. 28 wire on a cardboard tube 8 in. in 

 diameter and 18 in. long, taps being taken out 

 at each fifty turns. The secondary may he a 

 6-in. tube of the same length, wound with No. 36 

 wire an<l tapped at each 100 turns. A variable 

 condenser should be placed in shunt to the sec- 

 ondary terminals, for tuning, and a finely- 

 variable loading coil, or variometer, should be 

 placed in series with the primary. 



058 



